1. Field of the Invention
The present invention relates to an endoscope for oblique viewing and, more particularly, to an endoscope for oblique viewing having a visual field direction converting element.
2. Description of the Related Art
Endoscopes include endoscopes for oblique viewing having a visual field direction different from the endoscope longitudinal direction (also referred to as “oblique-viewing endoscope” below, “endoscope” in single form denoting an oblique-viewing endoscope). Oblique-viewing endoscopes include a side-viewing endoscope having a visual field direction perpendicular to the endoscope longitudinal direction, a forward-oblique-viewing endoscope having a visual field direction inclined toward a distal end, and rearward-oblique-viewing endoscope having a visual field direction inclined toward a proximal end portion. An objective optical system (also referred to as “objective system” below) provided in a distal end portion of an oblique-viewing endoscope has a visual field direction converting element having a reflecting function/refractive function to convert the visual field direction from the endoscope longitudinal direction to a predetermined direction. For example, each of objective systems 102 of conventional oblique-viewing endoscopes shown in FIGS. 1 and 2 has a prism P disposed as a visual field direction converting element immediately after a negative first lens L1, which is a one-side flat lens having negative refractive power (Japanese Patent Application Laid-Open Publication Nos. 7-294806 and 2004-226722). Oblique-viewing objective systems are longer in entire length than straight-viewing objective systems, and the ray height at the front side of the prism P tends to be higher. Therefore the prism P and the first lens L1 tend to be larger in size. In the following description of objective systems, “front” designates the side closer to an object to be observed and “rear” designates the image pickup device side.
In some case, in a straight-viewing objective system 202, as shown in FIG. 3, optical filters FL1 and FL2 are disposed in the objective system (see, for example, Japanese Patent Application Laid-Open Publication No. 10-113329). In the oblique-viewing objective system 102, however, it is difficult to dispose an optical filter because a space for disposition of a visual field direction converting element is lost if an optical filter is disposed in the objective system. That is, if the lens distance is simply set larger, the entire length of the objective system 102 is increased and the outside diameter is also increased. Also, the configuration of the conventional straight-viewing objective system 202 having optical filters is not an optimum lens configuration for an oblique-viewing endoscope and, therefore, cannot be directly applied to the oblique-viewing endoscope objective system 102.
Also, there is a demand for incorporating a thicker forceps channel in an endoscope. Realizing this while preventing an increase in outside diameter of an endoscope requires reducing the size of the objective system and an illumination optical system (also referred to as “illumination system” below) 3 (see FIG. 7).
In some case of a rearward-oblique-viewing endoscope, an optical specification such as setting the oblique-viewing angle (θ1; see FIG. 14) indicating the center of the field of view to a large value of 15° or setting the field of view indicating the scope of the field of view to a large value of 100° or more is required to improve the treatment performance. The above-described specification, however, entails an increased possibility of occurrence of a cut-off of a portion of the field of view, i.e., an image cut-off, caused by intrusion of a nozzle 4, a distal end hood of the endoscope, a forceps rising base 6 or the like (see FIG. 12). Increasing the placement distance between the forceps rising base 6 and the objective system is effective in preventing the occurrence of an image cut-off. However, the size of the distal end portion is thereby increased to produce adverse effects in terms of insertability and operability. Therefore, achieving both the prevention of occurrence of a image cut-off and the prevention of an increase in size of a distal end portion 5 requires, in particular, reducing the ray height at the first lens L1 of the objective system 2 and reducing the lens diameter.
In the oblique-viewing endoscope objective system 102, as shown in FIG. 4, a substantially large frame thickness is required for securing the desired strength of a lens unit frame (lens frame) on the side of a charge coupled device (CCD) 20, which is an image pickup device at the rear of the prism P. Longitudinal sectional views referred to below are each a schematic sectional view taken parallel to or along the longitudinal direction or the optical axis of an endoscope, while perpendicular-to-longitudinal-direction sectional views are each a schematic sectional view taken along a direction perpendicular to the longitudinal direction or the like.
In the conventional image pickup system, the image pickup element and the objective system 102 are so large that there is no problem with the sizes of the lenses and the prism P even in designing the lens unit frame if the sizes of the lenses and the prism P are determined mainly on the basis of the relationship with the ray height. However, a reduction in diameter of the objective system is required for achieving a reduction in diameter of the endoscope while reducing the size of the image pickup device and increasing the number of pixels. However, if the diameter of the objective system is reduced, the desired strength of the lens unit frame cannot be secured. Since increasing the lens distance between the first lens L1 and the prism P is required in the lens unit frame design for the purpose of securing the designed strength of the lens unit frame, the sizes of the prism P and the first lens L1 are increased as a result of increasing the lens distance. The ray heights at the lens and the prism P are thereby increased, so that the possibility of an image cut-off by the forceps rising base 6 for example is increased.
It is also possible to dispose an objective system 102 in which a prism P is provided immediately before a CCD 20 of an oblique-viewing endoscope 101 to bend the optical axis, as shown in FIG. 5 (Japanese Patent Application Laid-Open Publication No. 8-76028) and to dispose a straight-viewing objective system 202 in an inclined state without using a prism P, as shown in FIGS. 6 and 7 (Japanese Patent Application Laid-Open Publication Nos. 2005-287851 and 5-113541). In the oblique-viewing endoscope, however, the entire length of the image pickup system including the objective system-image pickup device structure is long and, therefore, it is not easy to reduce the diameter of the distal end portion 5.
In FIG. 7, a light guide LG and an illumination lens CL in the form of a lens having negative refractive power, constituting an illumination system 3 described below, are also illustrated. In some position diagrams or the like to which references are made below, an objective system is represented by a first lens L1 alone and an illumination system 3 is represented by a lens having negative refractive power CL. The diagrams not particularly specified are longitudinal sectional views in which a direction toward a distal end portion is indicated by an arrow (DE) and a direction toward a proximal end portion is indicated by an arrow (PE). Also, in some case, an illumination system and an objective system are collectively referred to as an optical system.
As described above, reducing the diameter of the distal end portion 5 requires a reduction in entire length of the objective system. An objective system having a reduced entire length is lower in optical performance and is incapable of securing a space for disposition of optical filters for color tone and image quality corrections, because the number of constituent lenses is small. Therefore, it is not easy to suitably design such an objective system in terms of optical performance with an image pickup device smaller in size and having an increased number of pixels.
Further, in an objective system of an oblique-viewing endoscope, since a prism P is provided, the possibility of occurrence of an angular deviation or the like is increased under the influence of variation in the optical path length or the angle of the prism P, and the influence of variation at the time of assembly on the lens unit frame as well as the influence of eccentricities of lenses. Therefore an oblique-viewing objective system needs optical adjustment of an angular deviation or the like at the time of assembly unlike a straight-viewing objective system.
In the lens configuration of an objective system ordinarily used in oblique-viewing endoscopes, a first lens L1, which is a negative lens, is disposed in front of the prism P, as described above. In this lens configuration, as shown in FIG. 9, a gap for adjustment is provided between a first lens L1 and a lens frame F holding the first lens L1, and a first lens L1 is moved in this gap in a direction perpendicular to an optical axis Z1 to make an optical adjustment of an angle of deviation or the like, i.e., an eccentricity adjustment.
After the optical adjustment, the gap for adjustment is filled with an adhesive to fix the first lens L1 in the lens frame F. However, there is a possibility of separation between the first lens L1 and the adhesive under the influence of heat in an external environment, the influence of a chemical solution at the time of sterilization of the endoscope, or the like. If separation occurs, moisture enters from the outside along the separated portions to cause dew condensation of water vapor on a lens inner surface and, hence, a fog on the lens inner surface such that an image to be observed is difficult to see. If the gap for adjustment is removed to avoid dew condensation, the objective system 2 varies largely in performance since optical adjustment cannot be made.
Reducing variations in performance of the objective system requires improving the accuracy of working on optical components including the lenses and the lens frame F, but mass production of high-accuracy component parts is not easy to perform. Also, it is difficult to perform the assembly process if no gap for adjustment is provided. Further, further improving the working accuracy is required for reducing variations of component parts having optical performances improved in correspondence with the reduction in size and the increase in number of pixels of image pickup devices. The first lens L1, which is a negative lens, is higher in refractive power than other lenses and is difficult to adjust finely.
Also, there is a possibility of considerable degradation in some other optical performance as a result of eccentricity adjustment to the first lens L1. In the case of an oblique-viewing objective system in particular, a partial defocus may be caused by an eccentricity adjustment made so that variation in the visual field direction is small and within a certain range in order to prevent the occurrence of an image cut-off. Conversely, if an adjustment is made to prevent a partial defocus, variation in the visual field direction becomes so large that the possibility of variation in the field of view and the possibility of occurrence of an image cut-off are increased.
Also, if the size of the first lens L1 in the objective system is increased, the distance between the objective system and the illumination system becomes so large that it is difficult for the illumination system to lightly and uniformly illuminate even a peripheral portion of the field of view for observation. At the time of closeup observation in particular, the illumination system cannot sufficiently illuminate in the field of view for observation; a peripheral portion of the field is left dark.
Further, in a case where the field of view for observation is made wider, that is, the field of view in the objective system is increased, for the purpose of improving observability, it is more difficult for the illumination system to sufficiently illuminate in the field of view and there is a possibility of a peripheral portion of the field of view for observation being dark.
The configuration of the distal end portion of the conventional oblique-viewing endoscope 101 is as shown in FIG. 7 or 8. A layout of optical component parts or the like in the distal end portion 5 for well balancedly illuminating in the entire scope of the field of view in a range of depth of field for observation is described, for example, in each of Japanese Patent Application Laid-Open Publication Nos. 2005-287851 and 5-113541.
In the conventional oblique-viewing endoscope 101 in which the illumination system 3 and the objective system 102 are disposed in this order from the distal end side DE, the entire region in the field of view cannot be well balancedly illuminated unless the illumination angle θ2 of the illumination system 3 is set equal to or larger than the oblique-viewing angle θ1 of the objective system 102, as shown in FIG. 10. At the time of closeup observation in particular, it is difficult to suitably perform illumination in a direction toward an upper region in the view (field of view) indicated by UP in FIG. 10. On the other hand, in the case of a disposition enabling suitable luminous intensity distribution at the time of closeup observation, it is difficult to suitably illuminate a lower portion of the view. Thus, the conventional oblique-viewing endoscope 101 may have a bad luminous intensity distribution such that a peripheral portion of the view is dark. Improving the luminous intensity distribution at the time of closeup observation requires reducing the distance between the objective system 102 and the illumination system 3. This is difficult to achieve because of the structure of the lens unit frame. In a case where the oblique-viewing angle θ1 is set to a large value of for example, 15° and in a case where the field of view for observation is increased to a larger field of view of 110° to 120°, it is further difficult to suitably illuminate the region in the field of view.
In the rearward-oblique-viewing endoscope in particular, the nozzle 4 and the forceps rising base 6 are disposed in the vicinity of the visual field direction and, therefore, the possibility of an image cut-off is increased due to the structure and the possibility is further increased if the oblique-viewing angle θ1 is increased or the angular scope of observation is increased.